In electrostatic printing and copying systems, an electrostatic writing head is commonly used to form a latent image of electrical charges onto a recording medium such as a paper web. The paper web carrying the latent image is then directed to a liquid toning applicator which deposits oppositely charged toner particles onto the paper web, thereby developing the latent image.
Such liquid toning applicators can be classified as pressure-type or suction-type. In suction-type applicators, a liquid pump is used to draw toner from a toner reservoir into the applicator and across one or more channels in the face of the applicator, wherein the toner comes into contact with the paper web. The flexible web serves to seal the face of the applicator thus permitting the pump, connected to the applicator outlet, to create a vacuum or suction in the channels and thereby draw toner into the applicator from a toner reservoir. The outlet of the toner pump returns the spent toner to the toner reservoir. In this manner a continuous recirculation of the toner occurs. The flexible paper web thus forms a liquid seal around the periphery of the applicator assuring that pressure differences are maintained. The face of the applicator must be oriented upward to insure that the toner is not spilled. If the paper web is pulled away from the applicator, damaged, or is simply depleted, the vacuum seal is broken, all liquid pumping ceases, and the toner flows gravitationally back into the toner reservoir. As a result, suction-type toner applicators are virtually leak-proof.
Despite the advantage of leak protection, suction-type applicators are severely limited by slow toning speeds. Since the paper web comes into contact with the toner in a channel of decreased pressure, the edges of the channel must support the web against the inward suction force. For this reason, the channels must be no greater than about a tenth of an inch wide in order to prevent the paper web from being pulled down into the toning channel and cutting off the flow of toner. To compensate for this narrow channel size, several parallel channels are incorporated into the face of the applicator. A practical design has these narrow channels aligned perpendicular to the direction of movement of the web. Such toning applicator systems are referred to as "cross-flow" applicators.
As the speed of the web passing over a cross-flow applicator is increased, more and more channels are required in order to insure that adequate toning occurs. As additional channels are incorporated into the applicator, the paper web is subjected to greater sliding friction. Furthermore, narrower channels, which can typically be up to 36 inches long, require stronger suction from the liquid pump in order to get the toner to quickly flow therethrough. This adds to the downward force on the paper web, and further increases the sliding friction. As web speeds approach 2 inches per second, the speed necessary for high speed plotting or printing, the number of required channels is so great that the sliding friction of the web against the applicator becomes impractical. Thus, suction-type applicators are restricted optimally to web speeds below approximately 1 inch per second.
Pressure-type applicators function in a slightly different manner. Instead of using a vacuum to draw toner across the face of the applicator, the toner is pumped out of a toner reservoir using positive pressure. In one embodiment of a pressure-type applicator the toner is pushed by positive pressure through a channel in the applicator and finally received by an inlet port on the opposite side of the applicator channel for continuous recirculation. Thus, pressure-type applicators do not create suction in the applicator channels, and sliding friction of the paper web is greatly reduced.
Since suction is no longer a problem, pressure-type applicators are freed from the "narrow-channel" geometries of suction-type applicators. For instance, the channel can be almost the full size of the applicator's contacting surface, extending across the entire width of the paper web, and the full extent of the applicator in the direction of movement of the web. One such "full-width" channel can be as effective as ten or twenty cross-flow channels, and almost totally without paper sliding friction. As a result, far higher web speeds can be employed without system compromises or image quality sacrifices.
However, manufacturing such "full width" channel applicators creates additional complications. When a broad surface such as the surface of the applicator is manufactured, geometric irregularities in the surface occur. That is, the applicator surface is not completely flat. Furthermore, pressure-type applicators are hampered by toner leakage. Even when precisely designed and constructed, toner leakage still occurs around the edges of the applicator. Often there is no attempt to minimize leakage, and the flow of toner is simply collected by a bucket or funnel situated underneath the applicator, and then recirculated. Using buckets increases the cost of the system, and the buckets also require occasional cleaning. Additionally, in applicator systems in which a single applicator is used for several different colored toners, the use of buckets results in color cross contamination due to toner mixing within the buckets.
As described in co-pending U.S. Pat. application Ser. No. 07/998,458, positive air pressure may be used to confine the toner fluids to the surface of the applicator in a pressure-type system. In such a system, air is introduced at the peripheral edges of the applicator surface, at a pressure greater than the pressure of the fluids confined within the applicator. The high pressure air prevents fluid leakage from the edges of the toner applicator, thereby eliminating the need for a funnel or bucket to catch spilled toner. In so doing, a difficult cleaning problem is eliminated, and a single applicator may be used for multiple color toning.
Unfortunately, the high pressure air used to confine the toner to the surface of the applicator pushes the paper web away from the toner applicator. To overcome the problem of web-applicator separation, the paper web may be held in place using a backing support which serves the dual purpose of keeping the web grounded during toning, as well as holding the web flat against the surface of the applicator. In pressure-type systems using pumps or blowers to create positive air pressure, the web must be maintained at a distance of no greater than 1-2 thousandths of an inch from contacting surfaces throughout the entire surface area of the applicator, otherwise the cost of supplying enough air to the applicator surface becomes impractical.
However, manufacturing two broad surfaces, such as a backing support and a toner applicator, which are mated to within 1-2 thousandths of an inch of each other is extremely costly. That is, forming a backing support which exactly matches the geometric irregularities in the surface of the applicator is cost prohibitive. Furthermore, lower cost mass-produced backing supports do not have the required accuracy necessary to precisely retain the paper web in close proximity to the applicator surface such that excess leakage of air is prevented.
Therefore it is an object of this invention to provide a backing support which is able to retain a recording medium in close proximity to an applicator, without requiring costly machining, and which will conform to variations in the surface of the applicator such that leakage of air from between the applicator and the recording medium is prevented.